This invention relates to fans designed to move air axially such as free-standing room fans or fans for moving air through a heat exchanger.
My U.S. Pat. No. 4,358,245 and my U.S. patent application Ser. No. 569,988 (filed Nov. 8, 1983), each of which is hereby incorporated by reference, describe various problems related to noise reduction and efficiency of axial-flow fans.
Specifically, the '245 patent discloses a fan with blades that are highly forwardly skewed along their entire length. In that context, skew is defined as the so-called "midpoint blade skew," the angle between a radius through the midpoint of the blade root and a radius through the midpoint of a chord at a given point on the blade. The high forward skew is designed to reduce noise, and, to accomplish that goal, the patent calls for a net blade skew angle (i.e., the mid-point blade skew from root to tip, see A.sub.b in FIG. 1) that is greater than 1/2 of the blade spacing. The fan disclosed in that patent has a net blade skew angle of 39.degree. and a blade spacing of 72.degree..
The '988 application discloses a fan in which the blade is rearwardly skewed to reduce noise and improve efficiency and compactness. The application is not concerned with the midpoint blade skew as defined above, but rather with the leading edge skew angle, illustrated in FIG. 1 as the angle A.sub.e between a tangent T at point X on the leading edge, and a radius R through the point. The leading-edge skew angle of the fan described in that application is 60.degree. (rearward) at the tip of the blade.
For many applications such as automotive engines and air conditioner condensers, various competing factors such as compactness, efficiency (e.g. power required), strength, and weight must be taken into account.